1. Field of the Invention
This invention is concerned with improvements in the manufacture of sheet materials having a relief effect wherein the relief is formed by selective decomposition of a blowing agent incorporated in the sheet.
2. Description of the Prior Art
In British Pat. No. 1,458,297 there is described and claimed a process for the preparation of a differentially expanded sheet material which comprises (a) applying an organic solvent to the surface of an expandable thermoplastic sheet containing a blowing agent and allowing the treated sheet to at least partially dry, (b) applying a composition comprising an ink and a kicker for the blowing agent to selected areas of the treated surface and (c) heating the sheet to a temperature at which the blowing agent in contact with kicker decomposes and retaining the sheet at this temperature for a suitable time so that those areas of the sheet in contact with the kicker are expanded to a greater extent than those areas not in contact with the kicker.
Kickers are well known in the art and their effect is to lower the decomposition temperature of certain blowing agents. If a kicker is in association with the blowing agent in certain areas only of the sheet, depression of the decomposition temperature of the blowing agent is obtained in only those areas. Hence a greater degree of expansion of those areas of the sheet containing kicker will be obtained than those areas not containing kicker when the sheet is heated to the depressed decomposition temperature for a given period of time.
Differential expansion of an expandable thermoplastic sheet material containing a blowing agent may be obtained by applying a kicker to certain areas only of the surface of the sheet before heating the sheet to achieve expansion. Application of a kicker to the surface of the sheet possesses attractive advantages however it was found that, when the kicker is applied together with an ink to the surface of an expandable thermoplastic sheet the adhesion of the ink to the thermoplastic sheet is inadequate during subsequent treatment of the sheet. In particular, when multi-colour printing is employed, ink from one printing station has a tendency to pick-off at a subsequent printing station or onto a path roller.
A significant improvement in the adhesion of the ink is obtained when, as described in British Pat. No. 1,458,297, an organic solvent possessing an affinity for the thermoplastic material is applied to the surface of the sheet before applying the kicker. The organic solvent is preferably applied in combination with a resinous binder as a lacquer in which case it will form at least part of the solvent phase. The advantage of applying a lacquer is that, with one coating, the thermoplastic sheet may be treated with the organic solvent and with one or more additional agents present in the lacquer such as a silica or silicate matting agent.